Happy to look into this for you. Is it a specific type of thread? How are you replying? Are you using the quick response option, or full reply? Are you replying from the thread itself, or clicking through on an email?
It happens it seems anywhere. I was replying to a PM it happened, trying to reply to a thread just by typing it in and hitting the "submit your answer" button, it happens like 5 times a day. No other of my sites suffer an outage, I have 100mb/s business class internet access, so it's not my net. I use google chrome.
Taking a look at some of the stats, it looks like we had a pile of these server errors crop up yesterday. Today seems smoother though. Probably just a server hiccup, but I'll have one of the devs take a peek and see if there's a deeper issue. Thanks for the heads-up on this.